<template>
  <ul class="list">
      <li class="item" v-for="item of letters" :key="item"
      @touchstart='handleTouchstart'
      @click="handleLetterClick"
      @touchmove='handleTouchMove'
      @touchend='handleTouchEnd'
      :ref="item">
          {{item}}</li>
  </ul>
</template>

<script>
export default {
  name: 'CityAlphabet',
  props: {
    Alphabet: Object
  },
  data () {
    return {
      touchStatus: false,
      startY: 0,
      timer: null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  computed: {
    letters () {
      const letters = []
      for (let i in this.Alphabet) {
        letters.push(i)
      }
      return letters
    }
  },
  methods: {
    handleLetterClick (e) {
      this.$emit('change', e.target.innerText)
    },
    handleTouchstart () {
      this.touchStatus = true
    },
    handleTouchMove (e) {
      if (this.touchStatus) {
        if (this.timer) {
          clearTimeout(this.timer)
        }
      }
      this.timer = setTimeout(() => {
        const touchY = e.touches[0].clientY - 79 // 距离可视区域顶部距离
        const index = Math.floor((touchY - this.startY) / 20)
        if (index >= 0 && index < this.letters.length) {
          this.$emit('change', this.letters[index])
        }
      }, 16) // 函数节流，结余handleTouchMovede的执行频率，提高网页性能
    },
    handleTouchEnd () {
      this.touchStatus = false
    }
  }
}
</script>

<style scoped>
  .list {
      display: flex;
      flex-direction: column;
      justify-content: center;
      /* 让字母垂直方向居中  */
      position: absolute;
      right: 0;
      top: 1.58rem;
      bottom: 0;
      width: .4rem;
  }
  .item {
      line-height: .4rem;
      text-align: center;
      color: #00bcd4;
  }
</style>
